Multicasting denominates a process in which one sender or transmitter sends the same message, respectively information content or data, to a plurality of receivers. In many cases, the message sent can technically be received by more receivers than the plurality of receivers addressed by the transmitter, particularly in radio-based telecommunication, which in principle is a broadcast medium. Multicasting is not limited to any particular transmission technology, but can be applied in any transmission system, e.g. for any kind of wireline or wireless technology. For mobile communication, multicasting has e.g. been generally described in 3GPP TS 26.346: Multimedia Broadcast/Multicast Service (MBMS); Protocols and codecs; Release 6 or in 3GPP TS 25.346: Introduction of the Multimedia Broadcast/Multicast Service (MBMS) in the Radio Access Network (RAN); Stage 2.
As a plurality of receivers takes part in the multicast transmission, the respective channel qualities may differ from each other. Such channel quality may be defined e.g. in terms of supported transmission rates, packet error rates, etc. This particularly applies—however not limited to this example—to mobile communication environments, in which different receivers (e.g. user terminals) experience different radio reception conditions, e.g. due to their distance to a transmitter (e.g. a base station), mountain effects, travel speed etc., resulting in different supported transmission rates of the different receivers. Additionally, the reception conditions, e.g. said supported transmission rates, may vary over time, e.g. due to changing environmental conditions or due to movement of receivers. This may, however, also apply to other kinds of transmission systems, e.g. IP-traffic, in which transmission data rates may be different or change due to access technologies (Ethernet, WLAN, etc.) and/or network load, which may be different for different receivers and/or vary over time.
For a multicast transmission, it is desirable and in some cases indispensable that all addressed receivers have, at the end of the transmission, obtained the complete message to be sent. If, however, the transmission data rate is adapted to the receiver having the least channel quality, possibly transmission resources are wasted or average data rate is kept at a lower level as necessary. The general goal is therefore to maximise the overall throughput in a multicast transmission, i.e. the ratio of the amount of transmitted data and the time or other resources needed until all receivers have received the data.
In some cases, e.g. mobile radio communication according to the LTE standard, transmission is slotted into transmission time intervals (TTI). The transmit rate can be chosen by the base station independently in each TTI based on the channel quality per user equipment, which is assumed to be known by the base station. The channel quality on the transmission link of user equipment (UE) u in TTI s can be expressed as the transmit rate ru,s that achieves a desired low reception failure probability (also known as block error probability, BLEP) at UE u. If in a certain TTI s the transmit rate Ras used by the base station is smaller than or equal to a supported transmit rate ru,s of UE u (Ras≦ru,s), then it is assumed that BLEPu=0, and else BLEPu=1, i.e. in the latter case reception always fails. The appropriate choice of Ras is also called link adaptation. Since the choice of Ras for TTI s also determines which UEs will fail to receive in TTI s, the choice of Ras can be interpreted as a scheduling decision, i.e. UEs that will fail to receive can be regarded as being not scheduled in TTI s.
Generally, the multicast throughput can be defined as the smallest throughput of all UEs, where the throughput per UE is the average over all TTIs, taking into account for each TTI either the Ras , if Ras≧ru,s or else zero rate. Alternatively, the data packets effectively received by the user equipment may be evaluated.
A method and apparatus for opportunistic multicasting with coded scheduling in wireless networks is disclosed in WO 2008/079222 A1. According to this document, the multicast transmission uses error correction coding with code blocks covering a virtually infinite number of consecutive TTIs. The assumption is a code with the property that a UE can decode a code block if it can receive at least a fraction c, also called the code rate, of the encoded bits of the code block. Such codes are also known as error correction codes or erasure codes. This allows the base station to select a transmit rate Ra so that UEs fail to receive a fraction 1-c out of the encoded bits, accumulated over all TTIs in the code block. In the case of MBMS (Multimedia Broadcast/Multicast Service), such coding can be e.g. the application layer coding defined in 3GPP TS 26.346: Multimedia Broadcast/Multicast Service (MBMS); Protocols and codecs; Release 6.
The approach of the WO 2008/079222 A1 is to set in each TTI the transmit rate Ras such that a certain number of L UEs can receive and the remaining N-L UEs fail to receive. Both iid (assuming that the supported transmit rates ru are independently and identically distributed) and non-iid rates ru are considered. For iid rates, in each TTI the UEs are sorted in the order of decreasing ru,sand the transmit rate Ras is set equal to rate of the L-th element in the sorted list. L is said to be a fixed value for the iid case, and may be determined numerically or by simulation of the system for different channel statistics. In the non-iid case, a modification of a Proportional Fair Sharing (PFS) algorithm is used, taking into account the channel quality and the scheduling history for each user. Again, the problem may be numerically solved, when channel statistics are known a-priori, or else by means of a simulation, based on which L can be determined dynamically.
The mentioned solution presents only heuristic approaches to solve the problem of scheduling in a multicast environment, which can not be guaranteed to yield at least near to optimal results, particularly when the channel statistics are not known.